This invention is directed to keyer circuits for use in electronic organs and more particularly to a keyer circuit for enhancing tones generated on electronic synthesis and formant organs.
The synthesis electronic organ is based upon the knowledge that sustained, complex musical tones can be synthesized by mixing properly scaled sine waves having frequencies representative of the fundamental and the various harmonics of the tone to be synthesized. This can be accomplished by having each organ playing key operate a group of contacts to connect the output signals from various harmonic generators to corresponding harmonic buses which are combined to form a synthesized tone. This is referred to as alternating current (AC) keying.
The formant electronic organ uses as starting signals so-called "bright waves" or signals which are rich in harmonic content including a fundamental frequency and a full complement of harmonics. Formant filter circuits which resonate or otherwise discriminate on a frequency basis are then used to remove unwanted harmonics and alter the harmonic balance of these complex signals to arrive at desirable tone signals. The formant system does not have the choice of tone coloration available with the synthesis approach, however, since it is not necessary to key the multiplicity of signals representative of the fundamental and various harmonics separately, fewer contacts are required in an AC keying system.
AC keying provides great control of tone quality since the level of the fundamental and each harmonic waveform is independently selected as desired. However, the multiple contacts of AC keying systems tend to make the playing key action too stiff for some people. Also, the multiple contacts needed in AC keying systems must be of high quality, since the tone components are switched at low signal levels. These high quality multiple contacts are quite expensive. In the standard type synthesis organ 61 sets of these expensive multiple contact switches are necessary which adds considerably to the overall cost of the organ. Furthermore, even high quality multiple contact switches are prone to failure at one or more contact points due to normal wear, dust and alignment problems and these failures result in the production of a distorted or harmonically incomplete tone signal.
The disadvantages of AC keying lead to the development of direct current (DC) keying wherein a DC signal controls a switching device which passes the AC signals while the device is activated. DC keying allows the use of a single contact per playing key and is utilized in both formant and synthesis organs.
An improved electronic organ DC keyer system is disclosed in U.S. Pat. No. 3,636,231 which provides the benefits of AC keying for synthesis organs but requires only one contact per playing key. Each keyer circuit comprises a plurality of keyer sections with one keyer section for the fundamental frequency and additional keyer sections for each of the desired harmonics of the fundamental frequency which make up the musical note or tone which is to be synthesized by the organ. By depressing the playing key and thereby closing the single contact the associated keyer circuit is operated.
Each keyer section comprises two transistors connected in series. Rectangular wave signals corresponding to the frequency of the fundamental and its harmonics are produced by a top octave generator and a series of dividers. One of the transistors of each keyer section is controlled by a selected one of these rectangular wave signals. A keyer input signal generated by the depression of each playing key controls all of the other transistors of the keyer circuit. Each keyer section produces a modulated rectangular wave signal output which is filtered to obtain an output signal which is substantially a modulated sine wave. Finally, the output signals from the keyer sections of a keyer circuit for a selected note are combined and passed to an output section to synthesize that note.
Of course, the DC keyer circuit of the above-identified patent can also be used in a formant organ. For use in a formant organ, bright waves are applied as tone input signals to the individual keyer sections. The keyer sections are similarly driven by a keyer envelope signal resulting in modulated bright wave output signals which are passed to formant filter circuits. The output signals from the formant filters are passed to an output section to sound the desired tones.
In keyer circuits for both synthesis and formant organs it is advantageous to provide an arrangement for controlling the tone envelope, i.e., the rate of attack and decay of the tone signal, to avoid transients which introduce "thumps" and other objectionable noise and also to achieve various desirable special effects. To achieve this purpose the keying signal from the playing key to a keyer circuit has a defined envelope, i.e., the rate of attack and decay of the keyer input signal is controlled.
Prior art arrangements provide only a single keying envelope signal to a given keyer circuit which controls the attack and decay time for the keyer input signal controlling the fundamental and harmonics or the bright wave signals. If a different tone signal is desired, it is known to change the keyer envelope signal; however, still only one envelope signal is applied to a keyer circuit at a time. For example, a keying envelope signal with a fast rise and slow decay time might be provided to the keyer circuit to generate a percussion tone. Although such envelope changes improve the tones generated, truly realistic tone production would require multiple keying signals with differing envelopes to be applied simultaneously to the same keyer circuit to independently control the attack and decay times for the fundamental and the harmonic components or the bright wave signals. Existing keyer circuits cannot be operated by multiple keying signals and, in any event, the use of such multiple keying signals would produce difficult timing and circuit design problems.
More particularly, many tones have components which have differing decay times, e.g., percussion tones wherein the initial strike components die away rapidly and the accompanying lower frequency components die away gradually over a substantially longer period of time. Such tones cannot be accurately synthesized by existing keyer circuits which receive a single keyer envelope signal.